Storage of potassium perruthenate solutions

I will have to prepare potassium perruthenate solution (15 mM in 0,05 NaOH). It will be used for oxidation of 5hmC. For one sample oxidation only 1 uL is needed, so more concentrated solution should be prepare. But what about its storage? Its better to keep this solution in +4 or -20? For how long this solution could be stored and used for oxidation? Thanks for any suggestions/advises.